DWDM optical communication systems have enabled significant progress in total capacity, flexibility and efficiency per single optical fiber. DWDM systems, which find use in long-haul or metro network applications, often employ multiple optical amplifiers (OAs), such as erbium-doped fiber amplifiers (EDFAs) or Raman amplifiers, in series to compensate for optical power loss in the fiber path and other optical components of the system. The OAs are normally operated under gain-saturation. When gain-saturated amplifiers are cascaded, however, a large spike or drops in output power occur when amplifiers experience sudden drops or increases in input power caused, for instance, when DWDM channels are dropped or added. The typical result is bit errors in surviving channels and can be significant for a large number of cascaded amplifiers. Srivastava, et al., “EDFA Transient Response to Channel Loss in WDM Transmission,” IEEE Photonics Technology Letters, vol. 9, no. 3, pp. 386-388, November 1997 and Karasek, et al., “Channel Addition/Removal Response in Cascades of Strongly Inverted Erbium-Doped Fiber Amplifiers,” IEEE Journal of Lightwave Technologies, vol. 11, no. 9, pp. 2311-2317, December 1998, recognize this problem.
Various optical and electrical techniques have been proposed in an effort to address this problem. For example, Srivastava, et al., and Karasek, et al., are directed to optical techniques and respectively propose a fast pump control in a two-stage EDFA and a strongly inverted EDFA with high pump power. These optical techniques are effective, but because they require new OAs, they are feasible only for new, so-called “green-field” optical communication systems. They are cost-prohibitive for retrofitting so-called “brown-field” systems that have installed legacy EDFAs.
Fischer, et al., “FEC performance under optical power transient conditions,” IEEE Photonics Technology Letters, vol. 15, no. 11, pp. 1654-1656, November 2003, proposes an electrical technique reported in public that uses forward error correction (FEC) to compensate for short burst errors. With this technique, it is possible to correct very short errors of perhaps a few hundred bits, but it is not suitable for practical applications, in which transients often lasts for at least thousands of bits. Kaneda, et al., “Polarization Mode Dispersion Tolerance of an Adaptive Threshold Receiver”, 2002 IEEE LEOS Summer Topicals, Mont Tremblant, Quebec, Canada, pp. 39-40, July 2002 proposes tracking the threshold voltage of a receiver's slicer circuit using FEC feedback. Unfortunately, this technique suffers a relatively slow response time resulting from the digital frame length and feedback scheme.
What is needed in the art is a better technique to compensate for output power transients. The technique should be faster and able to accommodate a wider dynamic range than those carried out in today's optical receivers. What is particularly needed in the art is an electrical technique to compensate for output power transients that is suitable for DWDM optical communication systems.